Urinary tract infections (8.3 million visits annually), urolithiasis (i.e., kidney and urinary tract stones; 1.3 million visits annually), interstitial cystitis (i.e., inflammation of the urinary bladder; 700,000 patients), and urinary incontinence (13 million adults affected, mostly older than 65) are also major health care problems. Individuals with ESRD must undergo renal replacement therapy, which includes peritoneal dialysis, hemodialysis, and kidney transplantation. Both peritoneal dialysis and hemodialysis, as their names suggest, rely on the ability to remove small dialyzable molecules from the blood—including metabolic waste products normally removed by intact kidneys—via diffusion across a selectively permeable membrane into a solution lacking these substances, thereby mitigating both their accumulation and associated adverse health effects. In addition, dialysis helps reestablish both fluid and electrolyte balance via removal of excess fluid, correction of acid-base changes, and normalization of plasma electrolyte concentrations). In peritoneal dialysis, the peritoneal membrane lining the abdominal cavity acts as a dialyzing membrane. Several liters of a defined dialysis solution are typically introduced into the abdominal cavity, and small molecules in blood diffuse across the peritoneal membrane into the solution, which can then be iteratively removed, discarded, and replaced. In hemodialysis, a patient’s blood is pumped through an extracorporeal artificial kidney in which blood is separated from a defined dialysis solution by an artificial semipermeable membrane that allows small molecules to diffuse from the blood down their concentration gradient into the dialysis solution, thereby removing small molecules associated with adverse health effects if allowed to accumulate in patients without functioning kidneys. Patients who are candidates for renal transplantation are often treated with dialysis until an appropriate donor kidney can be procured. Although anemia has historically been a significant problem in ESRD patients owing to severely reduced endogenous erythropoietin production, this problem can now be easily corrected in patients undergoing chronic dialysis via administration of erythropoiesis-stimulating agents (e.g., recombinant human erythropoietin).
CHAPTER 33 Elements of Renal Function 583 • Fig. 33.1 Structure of a human kidney, cut open to show the internal structures. (Modified From Boron WF, Boulpaep EL. Medical Physiology. 2nd ed. Philadelphia: Saunders Elsevier; 2009.) medulla in the human kidney is divided into conical masses called renal pyramids. The base of each pyramid originates at the corticomedullary border, and the apex terminates in a papilla, which lies within a minor calyx. Minor calyces collect urine from each papilla. The numerous minor calyces expand into two or three open-ended pouches, the major calyces. The major calyces in turn feed into the pelvis. The pelvis represents the upper expanded region of the ureter, which carries urine from the pelvis to the urinary bladder. The walls of the calyces, pelvis, and ureters contain smooth muscle that contracts to propel the urine toward the urinary bladder.
Blood flow to the two kidneys is equivalent to about 25% (1.25 L/min) of the cardiac output in resting individuals. However, the kidneys constitute less than 0.5% of total body weight. As illustrated in